Get a copy of the developer version of 2012, install it to a new server, restore a copy of the database, change your connection strings and test test test every and all functionality of the app, if everything works then fine, upgrade your dev test qa environments get business sign off then upgrade production, but ensure you sort all the leaglities out like licensing etc etc first.

Yes obviously, i will be getting licensed copy.But my question is my application is too much big, so whether i will have to test each and every form?Otherwise is any other solution for this. I am not aware about any functions which are not available in 2012 and that available in 2005.

Run the upgrade advisor, see if it shows any depreciated features or things which work in 2005 which dont work in 2012 like old style joins etc.

Then you really should test all your forms in your application as you never know what could happen, 99% of the app may function correctly, but that 1% if its a mission critical piece of code and you didnt test it I would not like to think of the consequences, to yourself or your company. But if its only a small piece of functionality that doesnt work and you can release a patch for it and it doesnt impact the company then you should be ok, but never say never as stranger things have happened at sea (as the say).